"Dieser Elefant ist extrem groß."

[NikolasV]

The adjective "extrem" becomes an adverb untransformed. Interessant.

[Katzenschlafs]

It seems that many German words are both adverbs and adjectives without changing form. I can't think of another example at the moment, but I know I've come across a few.

[af056]

I put gross and that was wrong.
Can anyone explain why sometimes the eszet is necessary and sometimes a double s can be used instead?